slowing down?

I was concerned about one of my snails this week- the horned nerite in the main tank. I hadn't seen it for several days, then finally saw it on its back under the heater. I moved it, placed upright, and watched it for a while. It did not move- for a day, then another. I lifted it out of the tank twice to check- no bad smell, and when gently poked it slowly closed the operculum. So it's still alive. Now it's started moving around again, slowly.

I've had this one for over a year, now. I did notice most of its 'horns' have worn off the shell- so it may be unhealthy? or lacking calcium again? will add another piece of cuttlebone- it's been a while.

The horned snail in the other tank is doing fine, incidentally. Those specks on the glass around it are baby malaysian trumpet snails. Looks like it's time to clean the inside glass, again.